irreversible reactions

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Irreversible reactions

Most chemical reactions are considered irreversible – the products that are made cannot readily be changed back into their reactants.

For example, when wood burns it is impossible to turn it back into unburnt wood again!

Similarly, when magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen, it is not easy to reverse the reaction and obtain the magnesium.


In an ordinary reaction; all reactants end up as products; there is 100% conversion

CONCENTRATION CHANGE IN A REACTION

As the rate of reaction is dependant on the concentration of reactants... the forward reaction starts off fast but slows as the reactants get less concentrated

FASTEST AT

THE START

SLOWS DOWN AS REACTANTS ARE USED UP

TOTAL CONVERSION TO PRODUCTS

THE STEEPER THE GRADIENT, THE FASTER

THE REACTION


What are reversible reactions?

Reversible reactions occur when the backwards reaction (products reactants) takes place relatively easily under certain conditions. The products turn back into the reactants.

+A B C D+

(reactants) (products)

For example, during a reversible reaction reactants A and B react to make products C and D.

However, products C and D can also undergo the reverse reaction, and react together to form reactants A and B.


Reversible and irreversible reactions

What kind of reactions are reversible and irreversible?


Hb + 4O2 Hb.4O2

Reversible biochemical reactions

Many biochemical reactions (those that take place inside organisms) are reversible.

There are also some very important industrial reactions, like the Haber process, that are reversible.

For example, in the lungs, oxygen binds to haemoglobin (Hb) in red blood cells to create oxyhaemoglobin.

When the red blood cells are transported to tissues, the oxyhaemoglobin dissociates back to haemoglobin and oxygen.


Heating copper sulfate


Heating ammonium chloride

An ammonium salt can be made by reacting ammonia with an acid. Some of the salt will decompose back into the reactants when heated.

ammonium chlorideammonia + hydrogen

chlorideNH3 (g) NH4Cl (s)HCl (g)+

NH4Cl decomposes back into NH3 and HCl gases when heated

NH4Cl reforms in the cooler part of the test tube


Initially, there is no backward reaction but, as products form, it speeds up and provided the temperature remains constant there will come a time when the backward and forward reactions are equal and opposite; the reaction has reached equilibrium.

EQUILIBRIUM REACTIONS

In an equilibrium reaction, not all the reactants end up as products; there is not a 100% conversion.

BUT IT DOESN’T MEAN THE REACTIONIS STUCK IN THE MIDDLE

FASTEST AT THE START

NO BACKWARD REACTION

FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP

BACKWARD REACTION

STARTS TO INCREASE

AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE

EQUAL AND OPPOSITE


IMPORTANT REMINDERS

• a reversible chemical reaction is a dynamic process

• everything may appear stationary but the reactions are moving both ways

• the position of equilibrium can be varied by changing certain conditions

Trying to get up a “down” escalator gives an excellent idea of a non-chemical situation involving dynamic equilibrium.

DYNAMIC EQUILIBRIUM

Summary When a chemical equilibrium is established ...

• both the reactants and the products are present at all times

• the equilibrium can be approached from either side

• the reaction is dynamic - it is moving forwards and backwards

• the concentrations of reactants and products remain constant


Simply states

“If the concentrations of all the substances present at equilibrium are raised to the power of the number of moles they appear in the equation, the product of the concentrations of the products divided by the product of the concentrations of the reactants is a constant, provided the temperature remains constant”

There are several forms of the constant; all vary with temperature.

Kc the equilibrium values are expressed as concentrations of mol dm-3

Kp the equilibrium values are expressed as partial pressures

The partial pressure expression can be used for reactions involving gases

THE EQUILIBRIUM LAW


for an equilibrium reaction of the form...

aA + bB cC + dD

then (at constant temperature) [C]c . [D]d = a constant, (Kc)

[A]a . [B]b

where [ ] denotes the equilibrium concentration in mol dm-3

Kc is known as the Equilibrium Constant

THE EQUILIBRIUM CONSTANT Kc

Example Fe3+(aq) + NCS¯(aq) FeNCS2+(aq) Kc = [ FeNCS2+ ] with units of dm3 mol-1

[ Fe3+ ] [ NCS¯ ]


for an equilibrium reaction of the form...

aA + bB cC + dD

then (at constant temperature) [C]c . [D]d = a constant, (Kc)

[A]a . [B]b

where [ ] denotes the equilibrium concentration in mol dm-3

Kc is known as the Equilibrium Constant

THE EQUILIBRIUM CONSTANT Kc

VALUE OF Kc

AFFECTED by a change of temperature

NOT AFFECTED by a change in concentration of reactants or products

a change of pressure

adding a catalyst


Reversible or irreversible?


True or false?


”When a change is applied to a system in dynamic equilibrium, the

system reacts in such a way as to oppose the effect of the change.”

LE CHATELIER’S PRINCIPLE


CONCENTRATION

FACTORS AFFECTING THE POSITION OF EQUILIBRIUM

The equilibrium constant is not affected by a change in concentration at constant temperature. To maintain the constant, the composition of the equilibrium mixture changes.

If you increase the concentration of a substance, the value of Kc will theoretically be affected. As it must remain constant at a particular temperature, the concentrations of the other species change to keep the constant the same.


CONCENTRATION

example CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)

the equilibrium constant Kc = [CH3COOC2H5] [H2O] = 4 (at 298K)

[CH3CH2OH] [CH3COOH]

increasing

[CH3CH2OH] - will make the bottom line larger so Kc will be smaller

- to keep it constant, some CH3CH2OH reacts with CH3COOH

- this reduces the value of the bottom line and increases the top

- eventually the value of the constant will be restored

decreasing

[H2O] - will make the top line smaller

- some CH3CH2OH reacts with CH3COOH to replace the H2O

- more CH3COOC2H5 is also produced

- this reduces the value of the bottom line and increases the top




Predict the effect of increasing the concentration of O2 on the equilibrium position

2SO2(g) + O2(g) 2SO3(g)

SUMMARY

REACTANTS PRODUCTS

INCREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE RIGHT

THE EFFECT OF CHANGING THE CONCENTRATION ON THE POSITION OF EQUILIBRIUM

DECREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE LEFT

INCREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE LEFT

DECREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE RIGHT

Predict the effect of decreasing the

concentration of SO3 on the equilibrium position

EQUILIBRIUM MOVES TO RHS

EQUILIBRIUM MOVES TO RHS


PRESSURE

When studying the effect of a change in pressure, we consider the number of gaseous molecules only.

The more particles you have in a given volume, the greater the pressure they exert.If you apply a greater pressure they will become more crowded (i.e. they are under a greater stress). However, if the system can change it will move to the side with fewer gaseous molecules - it is less crowded.

No change occurs when equal numbers of gaseous molecules appear on both sides.INCREASE PRESSURE MOVES TO THE SIDE WITH FEWER GASEOUS MOLECULES

DECREASE PRESSURE MOVES TO THE SIDE WITH MORE GASEOUS MOLECULES

THE EFFECT OF PRESSURE ON THE POSITION OF EQUILIBRIUM

Predict the effect of an increase of pressure on the equilibrium position of..

2SO2(g) + O2(g) 2SO3(g)

H2(g) + CO2(g) CO(g) + H2O(g)


MOVES TO RHS :- fewer gaseous molecules

NO CHANGE:- equal numbers on both sides


TEMPERATURE

• temperature is the only thing that can change the value of the equilibrium constant.

• altering the temperature affects the rate of both backward and forward reactions

• it alters the rates to different extents

• the equilibrium thus moves producing a new equilibrium constant.

• the direction of movement depends on the sign of the enthalpy change.



TEMPERATURE





• the direction of movement depends on the sign of the enthalpy change.REACTION TYPE DH INCREASE TEMP DECREASE TEMP

EXOTHERMIC - TO THE LEFT TO THE RIGHT

ENDOTHERMIC + TO THE RIGHT TO THE LEFT



TEMPERATURE• temperature is the only thing that can change the value of the equilibrium constant.




• the direction of movement depends on the sign of the enthalpy change.

REACTION TYPE DH INCREASE TEMP DECREASE TEMP



Predict the effect of a temperature increase on the equilibrium position of... H2(g) + CO2(g) CO(g) + H2O(g) DH = + 40 kJ mol-1

2SO2(g) + O2(g) 2SO3(g) DH = - ive



TEMPERATURE





• the direction of movement depends on the sign of the enthalpy change.REACTION TYPE DH INCREASE TEMP DECREASE TEMP



Predict the effect of a temperature increase on the equilibrium position of...

H2(g) + CO2(g) CO(g) + H2O(g) DH = + 40 kJ mol-1 moves to the RHS

2SO2(g) + O2(g) 2SO3(g) DH = - ive moves to the LHS



CATALYSTS


Ea

MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY

EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER

MOLECULAR ENERGY

NU

MB

ER O

F M

OLE

CU

ES W

ITH

A P

AR

TIC

ULA

R E

NER

GY

Catalysts work by providing an alternative reaction pathway involving a lower activation energy.


CATALYSTS

An increase in temperature is used to speed up chemical reactions but it can have an undesired effect when the reaction is reversible and exothermic.

In this case you get to the equilibrium position quicker but with a reduced yield because the increased temperature moves the equilibrium to the left.

In many industrial processes a compromise temperature is used (see Haber and Contact Processes). To reduce the problem one must look for a way of increasing the rate of a reaction without decreasing the yield i.e. with a catalyst.



CATALYSTS

An increase in temperature is used to speed up chemical reactions but it can have an undesired effect when the reaction is reversible and exothermic.

In this case you get to the equilibrium position quicker but with a reduced yield because the increased temperature moves the equilibrium to the left.

In many industrial processes a compromise temperature is used (see Haber and Contact Processes). To reduce the problem one must look for a way of increasing the rate of a reaction without decreasing the yield i.e. with a catalyst.

Adding a catalyst DOES NOT AFFECT THE POSITION OF EQUILIBRIUM. However, it does increase the rate of attainment of equilibrium. This is especially important in reversible, exothermic industrial reactions such as the Haber or Contact Processes where economic factors are paramount.



Opposing change

Whenever a change is made to a reversible reaction in dynamic equilibrium, the equilibrium will shift to try and oppose the change.

Increasing the temperature shifts the equilibrium in the direction that takes in heat.

Increasing the concentration of a substance shifts the equilibrium in the direction that produces less of that substance.

Increasing the pressure shifts the equilibrium in the direction that produces less gas.

Temperature

Concentration

Pressure

Condition Effect


Exothermic and endothermic reactions

All reactions are exothermic (give out heat) in one direction and endothermic (take in heat) in the other.

If the temperature is increased:

If the temperature is decreased:

equilibrium shifts to decrease the temperature

equilibrium shifts in the endothermic direction

equilibrium shifts to increase the temperature

equilibrium shifts in the exothermic direction


Opposing changes in temperature

Nitrogen dioxide is in constant equilibrium with dinitrogen tetroxide. The forward reaction is exothermic and the backwards reaction is endothermic.

What will happen if the temperature is increased? The equilibrium will shift to decrease the temperature,

i.e. to the left (endothermic).

If the temperature is decreased, more N2O4 will be produced.

N2O4 (g)2NO2 (g)

nitrogen dioxide dinitrogen tetroxide

More NO2 will be produced.


Concentration and equilibrium

Changing the concentration of a substance affects the equilibrium of reversible reactions involving solutions.

increasing theconcentration of substance A

equilibrium shifts todecrease the amount of substance A

=

decreasing theconcentration of substance A

equilibrium shifts toincrease the amount of substance A

=


Opposing changes in concentration (1)

Bismuth chloride reacts with water to produce a white precipitate of bismuth oxychloride and hydrochloric acid.

What will happen if more H2O is added?

If H2O is removed, more BiCl3 and H2O will be produced.

The equilibrium will shift to decrease the amount of water, i.e. to the right.

bismuth oxychloride

bismuthchloride water hydrochloric

acid+ +

BiOCl (s)BiCl3 (aq) H2O (l) 2HCl (aq)+ +

More BiOCl and HCl will be produced.


Opposing changes in concentration (2)

It will become more yellow.

Chlorine gas reacts with iodine chloride to produce iodine trichloride.

What effect will adding more Cl2 have on the colour of the mixture?

What effect will removing Cl2 have on the colour of the mixture?

It will become more brown.

iodinetrichloridechlorine iodine

chloride+ICl3 (s)Cl2 (g) + ICl (l)

pale green brown yellow


Pressure and equilibrium

Changing the pressure has an effect on the equilibrium of reversible reactions involving gases.

If the pressure is increased: equilibrium shifts to decrease the pressure equilibrium shifts in the direction of fewest

molecules

If the pressure is decreased: equilibrium shifts to increase the pressure equilibrium shifts in the direction of most

molecules


Opposing changes in pressure

Nitrogen dioxide is in constant equilibrium with dinitrogen tetroxide. Two molecules of nitrogen dioxide react to form one molecule of dinitrogen tetroxide.

If the pressure is decreased, more NO2 will be produced.

N2O4 (g)2NO2 (g)

dinitrogen tetroxidenitrogen dioxide

The equilibrium will shift to reduce the number of molecules, i.e. to the right (only 1 molecule).

What will happen if the pressure is increased?

More N2O4 will be produced.


Dynamic equilibrium and change


What is ammonia?

It is made industrially by reacting nitrogen with hydrogen in the Haber process. It is a reversible reaction, so it never goes to completion.

hydrogennitrogen + ammonia

N2 (g) 3H2 (g) 2NH3 (g)+

Ammonia is an important compound in the manufacture of fertilizer and other chemicals such as cleaning fluids and floor waxes.

Why is this a problem for companies making ammonia?


The Haber process


What is yield?

The amount of product made in a reaction is called the yield and is usually expressed as a percentage.

amm

onia

yie

ld (%

)

pressure (atm)

The yield of ammonia produced by the Haber process depends on the temperature and pressure of the reaction.


What is the Haber compromise?

In practice, though, these conditions are not used. Why?

The highest yield of ammonia is theoretically produced by using a low temperature and a high pressure.

A compromise is reached to make an acceptable yield in a reasonable timeframe while keeping costs down.

Lowering the temperature slows down the rate of reaction. This means it takes longer for ammonia to be produced.

Increasing the pressure means stronger, more expensive equipment is needed. This increases the cost of producing the ammonia.


Temperature, pressure and yield


Changing the yield of ammonia


N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1

Conditions Pressure 20000 kPa (200 atmospheres)

Temperature 380-450°C

Catalyst iron

HABER PROCESS


N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1



Catalyst iron

Equilibrium theory favours

low temperature exothermic reaction - higher yield at lower temperature

high pressure decrease in number of gaseous molecules

HABER PROCESS


N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1



Catalyst iron




Kinetic theory favours

high temperature greater average energy + more frequent collisions

high pressure more frequent collisions for gaseous molecules

catalyst lower activation energy

HABER PROCESS


N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1



Catalyst iron




Kinetic theory favours

high temperature greater average energy + more frequent collisions

high pressure more frequent collisions for gaseous molecules

catalyst lower activation energy

Compromise conditions

Which is better? A low yield in a shorter time or

a high yield over a longer period.

The conditions used are a compromise with the catalystenabling the rate to be kept up, even at a lower temperature.

HABER PROCESS


IMPORTANT USES OF AMMONIA AND ITS COMPOUNDS

MAKING

FERTILISERS 80% of the ammonia produced goes to make fertilisers such as

ammonium nitrate (NITRAM) and ammonium sulphate

NH3 + HNO3 ——> NH4NO3

2NH3 + H2SO4 ——> (NH4)2SO4

MAKING

NITRIC ACID ammonia can be oxidised to nitric acid

nitric acid is used to manufacture... fertilisers (ammonium nitrate)

explosives (TNT)

polyamide polymers (NYLON)

HABER PROCESS


The Haber compromise

To produce a high yield of ammonia, but with a fast rate of reaction and without the need for overly expensive equipment, the Haber process is carried out at 450 °C and 200 atmospheres.

The most important factor in deciding what conditions to use is therefore not yield, but total cost.

raw materials equipment

energy wages

What costs are involved in the industrial production of ammonia?


Maximizing productivity

What else can be done to maximise productivity in the manufacture of ammonia?

An iron catalyst is used to increase the rate of reaction. It speeds up both the forward and backward reaction, so the position of equilibrium is not affected.

The ammonia is cooled, liquefied and then removed as it is produced. This causes the equilibrium to shift to the right to produce more ammonia.

Unreacted nitrogen and hydrogen are recycled and given another chance to react.


Temperature, pressure and yield


Stages of the Haber process


Glossary

closed system – A system in which reactants and products cannot be added or removed once the reaction has begun.

dynamic – An equilibrium in which the forward and backward reactions take place at the same rate, so no overall change takes place.

Haber process – The industrial-scale process for making ammonia from nitrogen and hydrogen.

irreversible – A reaction that is impossible or very difficult to reverse.

reversible – A reaction in which the product(s) can be turned back into the reactants.

yield – The amount of product obtained from a reaction, usually expressed as a percentage.


Anagrams


Multiple-choice quiz